AttributeError: 'Context' object has no attribute 'features'

```
What steps will reproduce the problem?
1. compile a module that compiled fine with 1.4.1
2.
3.

What is the expected output? What do you see instead?
1.5:
pyang -f tree ietf-netconf-acm.yang
Traceback (most recent call last):
File "/ws/struong-sjc/download/pyang/pyang-1.5/bin/pyang", line 403, in
run()
File "/ws/struong-sjc/download/pyang/pyang-1.5/bin/pyang", line 316, in run
if m.arg in ctx.features:
AttributeError: 'Context' object has no attribute 'features'

1.4.1:
The tree is produced without any error.

What version of the product are you using? On what operating system?
Problem in pyang 1.5. Testing on Linux.

Please provide any additional information below.
Every module seems to fail with 1.5.
The way I test is simply download pyang (not install) and run python against it.
```

Original issue reported on code.google.com by `sht...@gmail.com` on 12 Dec 2014 at 9:12

Reproduce the failure with `pyang -f tree ietf-netconf-acm.yang`, then inspect `pyang-1.5/bin/pyang` around line 316 and the Context setup used by `run()`. Compare that path with the working 1.4.1 behavior; done means the command produces the tree without the `Context.features` AttributeError.

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